1. Field of the Invention
The present invention relates to a driver circuit for driving a group of driven elements such as an array of dot printing elements in a printer, or an array of dot display elements in a display device. The invention also relates to an optical print head using this driving circuit, and to an image forming apparatus employing this optical print head.
2. Description of the Related Art
One type of image forming apparatus having an optical print head is an electrophotographic printer in which the optical print head includes a linear array of light-emitting diodes (LEDs). The LEDs are driven individually to print dot images as described in, for example, Japanese Patent Application Publication No. 2008-44148.
The driver circuits that drive the LEDs in a typical LED print head are silicon monolithic integrated circuits (ICs) mounted end to end on an elongated printed circuit board, forming a row parallel to the LED array. Each driver IC drives a consecutive group of LEDs.
The driver ICs are secured to the printed circuit board with a thermosetting plastic adhesive that is cured in a high-temperature atmosphere. The curing process takes place in a natural state with zero stress as long as the printed circuit board unit remains near the curing temperature, but on return to a room-temperature environment, the substrate material of the printed circuit board contracts by a greater amount than does the silicon of the driver ICs. As a result, the driver ICs experience compressive stress in their longitudinal direction, parallel to LED array.
The amount of compressive stress differs from place to place within each driver IC. The differences in stress produce differences between the driving currents output from different parts of the driver IC, resulting in unwanted differences in the amounts of light emitted by different LEDs. The variations in light emission degrade the printing quality of the electrophotographic printer by causing uneven density in the printed output. A particular problem is a pattern of dark streaks that appears at low ambient temperatures at positions corresponding to the central parts of the driver ICs.